I would give a chance to small hole hay net
make sure you use good quality, green, leafy hay. if you can get analysis and look for moderate NDF content and high protein. fat in form of oil is the most calorie dense solution. you can use as much as 2 cups/day but you should build it up to that dose over 3-4 weeks, starting with few tablespoons. for building muscle mass and topline I would suggest Equistro Myopower.
Mare cubes or a product of a similar nature.
Copra does good on the coats too
I mix a high fat feed ( Tribute Kalm Ultra) with a high fiber feed (Tribute Kalm N EZ) for my hard keeper OTTB also going to be adding in a fat supplement(Tribute K Finish) for a little extra fluff during harder training months.
Use amplify. It worked to get weight back on my mom's thoroughbred