I think easiest would be using 2 pieces of styrene and filler putty. Glue a piece on the inside that covers the entire hole. Then cut out a piece that fits the hole, using a thickness that matches the thickness of the kit. Then use filler putty for the remaining tiny gaps and then sand when dry. 

Are you talking about the side sponsons, or the bottom of the hull. I don’t recall any holes in the bottom of the hull when I built mine a couple of years ago. But I covered the open sponson bottoms with sheet styrene.

I’m talking about the side sponsons.

Didnt know the correct term, thanks for clearing that up. I‘ve now corrected the name of this thread and also corrected the text in my original post.

The pictures you posted are a huge help.

You’re welcome. Most of the older Tamiya kits of this era were also offered in other boxings that were motorized, and the hulls had additional slots & holes for the motorization mechanisms such as the axle and on/off switch. This kit was a rarity from that time in that it did not. But Tamiya consistently used to leave those sponson areas over the tracks open allowing the see thru effect.

Grab one of those plastic 'For Sale' signs from the hardware store. For less than two bucks it's enough sheet plastic to block the sponsons on twenty-thirty tank kits.

Sometimes, the tags on the sprues with the Tamiya logo are big enough to use to make blanks. I've done this quite often before I had access to real hobby shops with sheet styrene.