My manual doesn't talk about retainers. It just instructs you to glue the bellypan onto the wing, removing the wing covering on the inside part of the belllypan.

Because it would be more easy to remove the if neccesary, I didn't remove the covering and just glued it to the wing using 30 minute epoxy. This worked fine for me, but is not the recommended way, since the covering may loosen over time, taking the pan with it. Best would probably be to remove strips of foil where the pan touches the fuse. I wouldn't remove the covering on the insid entirely, since their may be fuel or exhaust fumes getting in there.

My kit came with 6 wooden blocks, but those are ment as 'cowlings', blocks of wood you glue onto the firewall in order to attach the cowl using screws.

Using blocks to secure the pan will work, but adds extra weight to a plane you want to keep as light as possible, without any real reason to do so. If the Tojeiro will ever be in an incident where the bellypan might come off, trust me, it's a goner anyways. The fuse is quite weak right above the wing. At that point it mainly consists out of ~3cm of ~2mm plywood covered with balsa. If you land the plane too hard, or worse, crash it, the fuse will break in the middle.

Things you might want to reinforce are the landing gear mounting plate and the wing bolt plate.

The landing gear plate will work if all your landings are soft and smooth. If you have a less gentle touchdown (ie. cracks/bumps in the runway, or simply a hard tochdown) this plate will twist itself out of place, making the landing gear rip some of the hull.

The wing bolt plate might come off in flight (did so on my first Tojeiro, which Protech replaced) due to lack of glue between it and the fuse.