You'll want Scotch tape, not Sellotape. Try WHSmith's. Doubt the sensors will work through it though. May as well just turn them off if you're gonna cover them up.
I've got the exact same issue with my new Galaxy Active 2 44mm. I phoned the support and was basically told return it for a full refund or alternatively they could sell me a leather strap for £70! Considering the cost of the watch and the fact this has been an ongoing issue for soo long it seems incredible that there isn't at least a free alternative band for effected users. Never mind that Samsung can't be bothered to even provide a response to this threat it's just left ignored.
Irnonically if I do return the watch overall it will cost Samsung significantly more than the cost of a new strap given they will have paid two way shipping and need to recondition the watch. I'm fortunate I purchased it directly from Samsung if you buy from a 3rd party retailer you might not even get a refund.
Although not Ideal I have been able to minimize the rash returning..
1, Alternate wrists if you are wearing it over night for sleep monitoring etc. It may feel weird at 1st if you are used to it being on a certain side..
2, Clean the watch and strap with alcohol gel (standard hand gel works great) every week(or as often as you like, to prevent build up of germs etc under it.
So far ive only had the rash return once since following the above
Update on galaxy watch !!!!
After a long time switching wrists and getting a rash on both I noticed that the strap that came with the watch had raised embossed Samsung
Writing on the inside of the strap, I heated a spoon over the gas hob and removed the raised writing and for the last 3months I have been rash free.