I used a Ski Doo oil tank. Found a new old stock one on ebay. If you want to go this route look for one from a 1980s through about 2000 fan cooled engine...most were 503's sometimes called a 500.
The liquid cooled Ski Doos have a coolant tank attached and two extra ports for the rotary valve lube so you don't want those.
Note the sump to hold oil at steep angles so you don't need to mount the tank at an angle.
This one is off a 1996 Ski Doo Skandic 500. Be sure the cap is a oil tank cap...it is vented...the gas caps are not. It has a low oil warning switch you can wire to a light.