Just thinking about it, LW vass in Bedford used to offer this pumping glue stuff for land rovers and tractors etc to fill them with some kind of silicon to prevent punctures
builders foam won't work....tried it on a home made walk behind strimmer .....
had child bike wheels, always going flat, just biked the mess.....tried the foam, useless .....
ended up in my case with wheel chair tyres.....
for u how about a wheel barrow wheel convertion, pre foam filled....
My wheel barrow tyre kept going down, couldn't find a puncture, chucked soapy water over it and it was leaking at the rim. Knocked the tyre off the rim, wiped a bit of silicone grease over the tyre bead and put it back on.
No more leaks
Pu foam only degrades in sun light ,
£4 a can , might need two, might make the tyre a funny shape,
I would only use a proper gun though not the ****e disposable gun ,
Drill one hole to fit nozzle through and another to let the excess out ,