As already mentioned film holders are the way to go. Another option to consider, especially to practice, is to load them with photo paper and shoot that as a negative. With pinhole you are going to use the camera on a tripod anyway (or other support) so the very slow speed, ISO 3 to 6 is generally not an issue. For loading and developing the paper can be handled under safelight, is cheap, and can be contact printed or scanned into a file.
For those of us with a darkroom background and lots of photo paper laying about plus the chems and other equipment on hand shooting paper was a no brainer. If you have to start from scratch and buy everything then figure spending about $250 to get started. Depending on how well you can scrounge stuff up this could be less.
The downside is that paper is not sensitive to red and has limited latitude. It can be difficult to control contrast on bright outdoor scenes.
About 1/4-20 tripod holes. Because of the very light nature of my homebuilt 4x5 pinhole and lensed cameras I’ve never had a problem with tapping the 1/4-20 right into the hardwood, usually oak. Sounds crazy I know, but I’ve never had one strip out.
edit; Ah yes, I almost forgot, you will need to find, or cobble together, a dark space in your home. Fortunately I have a windowless laundry room that serves that purpose….with a dark towel covering the door bottom. Running water is a plus. Sometimes working in the middle of the night with all house lights off is an option, especially for insomniacs like myself.