How you deal with fox is you have to know what kind of pressure he's dealing and what to do about it. If you cc a Dair, you can bring up shield at any time. the moment when you'd SDI something is when you roll away or behind him. Dtilt beat a ton of foxes moves and even if it trades, it's good to give yourself breathing room. Spacies MU is really a combo or be combo-ed kind of fight. Getting one grab can be death for Fox if you're prepared for any option he chooses to get away. You said he was shining you out of Uthrow Usmash? Either he's frame perfect or you're not fast enough or both, but the moment he did that, you should then choose to go Uthrow Utilt instead because Utilt is faster and it can lead to a Nair. Also, watch your percentage, at 85%~ you're in danger of a Usmash death, switch your play to defensive. The key is to know when to tank, when to shield, when to space and when to go all in. Here's a little trick I learned recently, maybe I'm scrubby for not realizing, but I just figured out you can reverse a grounded yoshi bomb and the initial hitbox will come out behind you. So (really in any MU) as an example you could Dair on them/their shield. If you get a Full or near-Full Dair, then land behind them and Lcancel DownB, You catch them or you get a shield break.
As for advice on Fox, earn what you need to do (grab, zone off the ledge, etc.) and how you're going to do it to this particular player. I think one of the reasons this Mu is easy for me is because of my mental game and my reliance of outsmarting the opponent. Get fox into a rock-paper-scissors situation and cover as many options as you can (eggs let you pressure correct options for free, just toss one to cover a roll-away and they won't choose it). Basically, get him on the ground so you can call him out on his tech. Lastly, I hope you were using Rising Nairs, they are insanely useful in the MU.
Also, I had trouble because both DK and Ganon MUs require dynamic aerial spacing that I frankly can't do on keyboard. I need a mayflash so bad.