Have some automobile battery acid on-hand in case of a spill. I bought a gallon of 37% battery acid to have here just in case before I did this.
While I agree with very nearly 100% of what PE says, I have to say that I disagree with the suggestion that 37% sulfuric acid be used in the case of a 50% or other concetrated acid spill. 37% sulfuric is still very acidic and I'd suggest using something weaker, like vinegar or dilute (i.e. 10% or less) hydrochloric or sulfuric acid.
Using an acid this concentrated will invariable release even more heat and fumes and it will not make cleaning up easy. I've seen people in the lab I worked at use concentrated bases to neutralize concentrated acid and get burned much worse than the acid alone would have caused. It's the same principle at play here that should be avoided.
To clean up a concentrated hydroxide spill, I would suggest the following:
Solutions of sodium or potassium hydroxide greater than 10% have a contact rating of 4 - that means they are extreme hazards (i.e. they are corrosive). If you have contact during a spill, you should remove all contaminated clothing and then flush the contacted skin with large amounts of water.
For cleaning up the mess, you should use the folowing protactive equipment - goggles or a face shield, an apron or some old clothes, and proper gloves. Make sure the area is well ventilated.
You should contain and recover any spilled liquid when possible - wiping up with paper towels or spill pads will work well here. When the bulk of the spill has been contained, the residues from spills can be diluted with water and then neutralized with a
diluted acid - dilute acetic, hydrochloric, or sulfuric will work here. Absorb neutralized caustic residue on clay, vermiculite or another inert substance and package in a suitable container for disposal.
See this MSDS for 50% sodium hydroxide for more info:
http://www.jtbaker.com/msds/englishhtml/s4037.htm
I do completely agree with PE about being careful when preparing 50% hydroxide solutions. I would suggest only using Pyrex or other lab-type glassware as regular glass is just too likely to break if the solution gets too hot or too hot too fast.
Do use very cold water, but I'd also suggest having a water bath of very cold water to set the container that you are mixing the solution in into. Add a few hydroxide pellets to the container and stir it continuously with the container sitting in a cold water bath. When thats dissolved, add a few more pellets and continue mixing. As the water bath begins to warm up, dump it out and refill the bath with cold water. This is another reasons to use a Pyrex-type container as it needs to be able to potentially hold a hot solution while sitting in a cold bath of water. Regular glass containers and plastic may not be able to handle these conditions. When the solution is mixed transfer it to a HDPE plastic container for long term storage.
Another tip - do not simply dump all your hydroxide pellets into the container all at once. It will not only get very hot, but the pellets will simply sink to the bottom of the container and glue themselves together and make a big, hard brick of hydroxide that will be a very big pain the butt to get dissolved. (I have not done that personally, but I did see someone else to that... and I ended up cleaning that lump of hydroxide out of the container.)
If you are going to mix large amounts, I'd suggest simply buying some. You can buy sodium hydroxide that's already at 50% concentration. After making up a gallon of 50% hydroxide one time, I've decided that I have better uses of my time and I don't like to take the risk that's involved in making large amounts of hydroxide solutions.