Does anybody make such a thing?
I do my own 50hr checks, with an A&P/IA colleague (not least because removing the cowlings is a 2-person job) and while we try to pick a weekend when it is not pouring with rain, we aren’t always lucky, and the wintertime can be horrid. It cannot be done in a hangar, for airfield political reasons.
Obviously there are tents but they are hard to erect especially if it is windy. It needs to be big enough to cover the engine area, and a bit of the cockpit.
With a 3-blade prop, one could even take advantage of two of the blades to support something…
The internal room needs to be just enough to get in to remove the spark plugs, oil filter, etc.
Unfortunately I know nothing about a tent other than to think its innovative idea!… If winter weather is an issue, tent or no tent, one (admittedly slow!) approach to that issue is to incrementally move the annual to summer by playing the 13 month, 1st of the month sign-off game. I feel very fortunate to have a comfortable, dry place for maintenance.
The Annual must have a hangar (can’t jack up a plane outdoors, for example, plus security) and my previous facility is back online, or so I understand. The only drawback is a total lack of heating… moving the Annual to the summer is not really an option, due to downtime. However, thinking about it, we get it done in 4 days so maybe we should do that! OTOH there is something particularly satisfying about doing the Annual in a hangar, with OVC002 +TSRA 220/50G75 outside for the whole time 
The tent is for the intermediate services, for which we could also use the hangar but it is more of a hassle – a lot of driving for one of us two, and the weather risk. If I can’t fly back out, and land back at base, it is a hotel or a £80 taxi (x2).
Obviously there are tents big enough for whatever purpose you want but a conventional one big enough for this would probably be hard to erect and keep stable, as it would be sitting on concrete so no stakes could be driven into the ground. I was thinking one could use buckets filled with water to hold it down (there is an outside tap nearby).
TARPAFLEX do a range of temporary shelters, car ports etc that could be modified to fit in the way you describe. To hold something down the aircraft itself is pretty heavy – so why not devise a way of tying it to the airframe e.g. remove cowlings, position shelter and then tie the ridge of the shelter to the lifting ring on the engine with bungees. Of course if the aircraft is in its usual parking spot on concrete then ragbolts will hold anything down. Ragbolts can be countersunk so the ring does not protrude above the main concrete level – very discreet.
As mentioned, when properly timed, “moving” the annual by a month every year does not mean any downtime. Just start it 1-3 days before the end of the month.
Also, if Shoreham is so “police-like” as to surveil if you are doing maintenance in your hangar, then they will just as much tell you off once you make your first oil change on the apron…
A gazebo would be quite stylish! One sllghtly larger could be planted just behind the trailing edge, thus covering cabin and engine. Tie it down to the aircraft. Headroom could be tricky though.
Also what exactly is a 50hr check? Clearly not a legal requirement but includes an oil change I presume, but have you devised your own checklist? Perhaps a subset of the 100hr/Annual checklist from the Maintenance Manual?
Many (most?) airplane maintenance manuals have a 50h check as the smallest regular maintenance check.
Ah, yes… You are quite right! I haven’t racked up 50hrs yet…
