For HI, his HPBW is on the order of +/- 10 degrees. He's probably fine with 1 deg pointing accuracy which it looks like he may achieve based on his video. He's certainly ahead of most of us for the ability to say track M33 for 6 hours in a single night instead of compiling 18+ main beam transits for a really high SNR detect of the spin doppler peaks. The more I see from DIY solutions for rotating dishes of that size (or larger), the more I start to respect the price point of the RF HAM Design (marketed) solutions and/or the Yaesu G5500 solutions for mesh dishes up to 4.5m. He could probably just buy one of those to achieve his desired pointing accuracy. He's seems pretty serious and given the size of the radome, I'd be surprised if he doesn't try to squeeze a 3 or 3.7m in there. If I already owned the radome, I'd just buy the RF Ham Design mesh dish to put in there as a starting point, but he can get away without mesh given he doesn't have to deal with wind loading.
His whole mantra is "save it for parts" though, so he likes not using the intended equipment and seeing what can be done with scraps. This is one where getting a scrap radome at really low or zero cost opened a lot of possibilities and I'd look at COTS dish stuff for inside it.