one of the best things is mothballs..
there are two types; "old fashioned" (naphtha based) and chloro-benzine based.
https://en.wikipedia.org/wiki/Mothball
I prefer the naptha based.
both cost about $1 to $4 a box (usually about 2 at walmart or dollar store).
WM has naptha, usually.. dollar stores generally the CB based.
typically i'll put what i don't use in a double heavy zip-lock & stow it..
to start with i'll crush about 6 of them & sprinkle the powder throughout the vehicle..
this is pungent stuff, & the smell will be with you for a while, so you might want to experiment, first.
this stuff keeps out spiders, roaches (btw, boric acid is cheap (dollar store) & an effective passive deterrent), ants, fleas, etc..
not 100% effective, by any means, but definitely an active deterrent..
discovered this when i put my 1974 dodge xplorer in outdoor storage in florida..
figured that if it was a deterrent to moth & larvae, it should be broader spectrum to some extent, and it was.
now.. admittedly i used at least on box, & maybe 2, but after 6 months of months of warm, humid storage, i came back & there were *no* fucking ants, *no* fucking roaches & no spiders, either.. maybe a couple of webs, but nothing serious.. & the damn thing still smelled like the round little bastards, so it worked..
also have had a number of dogs in my vehicles & no infestations..
i'm not keen on folks who can't care for dogs or animals they carry, so don't feel bad if you limit the rides you can give them.. i mean, it's sweet to travel & to help, but there's a minimum responsibility people need to accept in order to actually travel, rather than just usurp..
naphtha is reactive.. not highly in its solid form, but it can react slowly with plastics & fabric, book covers..
.
the powder isn't a big deal..
i actually throw whole balls into corners.. about 10 or 15 after a bit of powder..
it's a cheap experiment.. i suspect it fucks with metamorphal development in any insect.
in any case, read the wiki & good luck..
C
i've picked