Who did you buy the Indian made one from ?
I've bought/fitted a number of single seats over recent years, and not had a problem.
They are big and heavy like you say though - the 'universal' type was like that.
It has that extra metal bar under the back of it.
The original british made ones used "leatherette" as the seat cover material, and it wasn't brilliant either.
I sourced some leather made ones, but that source has dried up. They cost a bit too.
Time will tell how they last.
Seat covers are still about, in various qualities, is there someone in Ireland who supplies classic type bits.
I'd avoid those springs that look like they've had a bad trip to the hairdressers !
I sourced some seat springs for a Norton recently - Indian made.
They were 4 & 1/2" inches, whereas the set I had set aside was 7" - and way too long.
Seem OK so far ...
Genuine black leatherhttps://www.ebay.com/itm/224508149388
Cheaper ? (seat cover) by far ... And a fairly reliable supplier in my opinion.https://www.ebay.com/itm/254528784531
You have to know that there is likely only one or 2 suppliers in India, and all the rest are what I'd call 'mark-up' resellers.
If you can find the original supplier, the prices are often really really inexpensive.
P.S. It goes without saying that I have not personally inspected anything that anyone might send you,
so as always caveat emptor and all that ...