The Ultra-Fab Protective Cover for Electric A-Frame Jacks - Vinyl - 12" x 14-1/2" # UF38-944020 is the protective cover I recommend for your Husky Brute 4,500 lb Trailer Jack # HT87247. I've attached a photo of the dimensions when laid flat, but I've also attached a video to assist with the fit. It has a durable vinyl construction with integrated draw string closure.
This is a Tandem Axle Steel Diamond Plate Trailer Fender. This Dual Axle Fender is made of 14 Gauge Steel. This Fender measures 10 inches wide x 72 inches long x 13 inches tall. If you're looking for a Heavy Duty Steel Fender for your tandem axle trailer this is the one for you. Most commonly this style of fender is welded to the trailers frame at several points, or to brackets that are welded to the trailers frame. Both sides of this fender are able to be used as a welding surface.
For a cover to use on the Bulldog Powered Drive Trailer Jack # BD500199 referenced in your question, I recommend the Ultra-Fab Protective Cover for Electric A-Frame Jacks # UF38-944020. This will easily slip over the jack and can be secured using the drawstring. This cover measures 12 inches x 14-1/2 inches. I have attached a video demonstration of the cover for you to check out.
According to the information I was able to find, the Ultra-Fab Electric A-Frame Jack you referenced, part # UF38-944016 would be a good choice for the Keystone Outback Toy Hauler you mentioned. The jack has a 3500 lb lift capacity, which is plenty for the listed tongue weight of the toy hauler, plus anything stowed on it. Among the many features of this jack, I like the fact that is uses LED lamps for long life as well as the fact that a 9/16 inch socket wrench can be used in an emergency...
One pretty awesome solution is the Towing Organizer by GR Innovations. This made in the USA organizational solution attaches around your trailer jack (either manual or electric) and gives you a place to hang two safety chains, an emergency brake cable, and an electrical harness. Making everything neat and organized (and making Marshall a very happy camper).
Replaces the bottom housing kit for Husky Brute Series HB4500 and HB4500R electric trailer jacks (HT87641 and HT87247). Kit includes covers to protect both the gearbox and the motor assembly from Features: Replaces the bottom housing kit for Husky Brute Series HB4500 and HB4500R electric trailer jacks Fits HT87641 and HT87247 Included covers protect both the motor assembly and the gearbox
Keep your trailer in line with this system that uses a telescoping bar with friction pads to correct trailer sway. Built-in grease zerks in the head provide simple maintenance. Includes an adjustable maintenance Powder coated steel resists corrosion Snap-up lever eases hookup by reducing lifting required by trailer jack System includes head assembly, adjustable shank, 2-5/16" hitch ball