Can try looking at padded zip bags meant for long stuffs like Martial Art Weapon or musical instrument or photography equipment or Artwork Stand etc.

Best will be shops selling bags for carrying weapons like airsoft or real rifles but they are very rare, if not not available in Singapore
Maybe Arts Friends have something

if not got to ask around for people who partice martial arts (like Wushu?) or Music for shops that might sell such bags.
Or you can get someone who can sew well to make one for you using materials for making bags and a few layer quilt blanket paddings.

Can also add a few pieces of Hard Sponge (meant for packing of expansive equipments or new appliances or for making seat cushion) at certain position to prevent the rifle from moving around.