Method "A" ...
Buy a Hartz Flea collar.
Cut collar in half
Put half in the heater vent & the other half in the fridge vent
Go and buy a roll of expanded metal "gutter guard".
Comes in 6" x 20" or 50' lineal foot rolls. Don't use the plastic version (at least not on the heater vent. The actual spec is called 1/4 #18 or 1/4" #20 (1/4" swd (short way of diamond), 18 gauge sheet) G60 galvanized or aluminum.
You have four basic entry points for Wasp (a.k.a. Mud-daubers(sp?) if your from up North ....at least thats what my wonderful & lovely yankee wife calls them). Range/stove vent, furnace vent, fridge vent, heater vent....
. Range/stove vent and furnace vent are most likely the culprit if you have wasp inside your unit.
Measure for your particular units .... but most post 2005 units are as follows (material cuts with household scissors);
Stove discharge vent is 4-1/2" (h) x 11" (L) .. I usually take the full 6" roll width and fold it pback on itself at the 4-1/2" mark with a 60-degree return bend (the return bend makes it stay in the vent cover easier).
Rear heater vent is 5" (H) x 7"(W)
(you will need three trash bag twist ties, cut-in half, to install. Strip the plastic off the wire ties to expose the metal (eliminates melted plastic fire hazard). Open the vent cover and attach the gutter guard ppieces from the inside using the trash bag ties.
Fridge vents; Remove the cover. The slat openings are actually horizontal ... so cut the expanded metal gutter guard to length and form the gutter guard over the openings from the inside & then Duct/HP tape the guards in place from the edges. Hospital-edge the rounded corners on the ends of the slat & tape over to seal.
Furnace vents; cut 3" OD circles and cut 4 more trash bag ties in half & strip the plasti for fastening. These covers will mount externally with the trash bag ties.
If this is too much of a DIY,, you can also buy pre-made mesh covers
for he penetrations and vents ...... but the methods mentioned above will cost you about $6.00-$8.00 either way, and take under an hour to make/install.
As an alternate to the expanded metal, you can also use the galvanized "hardware cloth" that they sell in any DIY store (spec is 1/4 X 1/4 A 23 GA, gaw ... but most stores sell it as 1/4", .025 welded, galvanized after, or "utility" cloth).Personally I find the expanded metal diamond pattern easier to work with & you dont have to worry about lining up squares to reduce the opening.