(P. S. - I cringe at the thought of someone getting gels from the theatre our theatre group performs at because the House gels have saved our budget in the past. I'll try to put you off by saying that those gels have been used quite a bit in front of very hot & bright lights so their transmission will vary more than brand new gel and it'll be less scratched up too).
Good point. You definitely do not want a used gel. What I meant was to see if a local theatre can order a sheet for you or sell you part of a sheet they have already.
I have just had a look through my book to see if there is a suitable amber colour. Golden amber 134 has a little bit of everything down to 300nm so would not be any good and deep golden amber 135 has a similar response to the reds but tails off into the 525nm region. Ilford's data sheet shows that MG IV has its sensitivity trailing off just above 550nm so whilst on paper the golden amber 135 does not look like it will work, in practice it may be o.k.
Obviously testing is the way to go with this type of thing but that is easier said than done if you don't have the samples.
If I remember, I will take my sample book home (it's at work now) and test some pieces next time I am in the darkroom as I have been thinking about making an amber safelight myself using this method.
Steve.