I've done a lot of pinhole photography many years ago. Got the idea from an article in RMC, where the author did something like you did. I took a different route, using a Vivitar 28mm lens that was totally manual and that I was no longer using. I made the pinhole in a strip of shim brass with a small drill, then sanded down around the hole to make it even thinner. Blackened it with a felt pen. The rear element of the lens was easily removed, and I used CA to glue the strip just before the lens' aperture blades. It worked the same as yours does, view and compose with the lens wide open, close it down so light only passes through the pinhole aperture.
I was using Canon film cameras at the time (AE-1, AE-1P, later A-1's), shooting both black and white and color slides. I found that doubling the exposure given by the camera seemed to work out about right most of the time.
A friend who used Pentax film cameras made a pinhole in a disk of thin sheet brass and taped it to the rear element of one of his lenses, it worked good for him too. It did not work on my Canon lenses. The problem with this method is he needed to view the scene through the pinhole, and the image was quite dim. He'd place a dark towel over his head to keep the light out.
I had a lot of success with my pinhole lens, having many of my photos printed in the model press in the mid-late 1980's, including several cover photos. One pinhole shot won me first place in Model Railroader's annual photo contest, March 1989. When I took similar scenes with the pinhole lens and with a regular Canon lens, the Canon shot was always sharper, but looking at the pinhole photos they seemed OK on their own. I later used Canon 24mm and 28mm wide angle lenses for my close up photos.
When I switched to Canon digital SLRs about 10 years ago, I could not get the same results with the pinhole lens. Canon cameras had (and most consumer grade ones still do) sensors that were smaller than a 35mm frame. So only about 60% of the light from the center of the lens is recorded by the sensor. This made the pinhole shots way less sharp than I achieved on film.
My partial solution was to use an extreme wide angle lens for my similar close up shots. I use a Tamron 11-18mm lens at 11mm, shooting at f/22. I can't get in as close as I could with the pinhole lens, but I just crop out the foreground and they're pretty good. I duplicated my contest winning photo from MR digitally, and it is pretty close.
Hope you have as much fun as I did using my pinhole aperture equipped lens!
Here's one of my pinhole photos, taken on my HO scale shelf layout 18" by 6':