To add on what others have said. If you rent from an off site vendor and need to use their truck/trailer it takes about an hour round trip to leave The Fort, bring the cart back to The Fort, bring the truck/trailer back, and get back to The Fort. So you can cut that about in half if you have you own way of getting the cart. Also It's about an $ 8-10 lyfy/uber each way to the pickup/drop off point. So figure an additional $40 to the total cost of off site.
I have stayed at The Fort 3 times. First time for 6 nights, and 2 times for 9 nights. The 6 night trip we used internal bus transportation but the other two trips we rented a cart from Kenny.
As others have said for a short trip like yours, I would rent right from Disney. Disney carts are somewhere around $65 a night and I want to say Kenny is about $40 a night for short stays (he discounts for longer stays). So Figure Disney will be around $195 and Kenny will be $120 and if you need Uber/Lyfy you are talking $160. So it's only a $35 savings and you also get 2 more hours at The Fort or the Parks.