You can always use boards in an attic to lie on / reduce the risk of going through the ceiling / plasterboard. 18mm chipboard will do the trick for many people as will thick ply (need to test according to your weight if heavy). So long as the board is able to fit through the loft hatch and long enough to span between beams, then it's ok. Just remember NOT to stand on any end overhanging the beams or the board will tip and you'll go straight through. To avoid this best, you want some overhang to avoid the risk of it sliding off the beam, but not a lot, to reduce the risk of overstepping the truss. Another tip could be to put a cross piece on the underside of each end to stop the board sliding. However, here you are relying on there being even spacing between the beams. You could also use hazard tape to mark the overhangs. Most people will use a board to lie on, however, with a couple you can use them to cross beams by standing on one and laying the other forwards and repeating until at the desired point. Alternatively if wealthier, you could floor / part floor the loft before starting. This again can be done either with proper flooring or chip cut to length. Mine is 1/2 floored with 18mm chip. It's not perfect but it works. However, I'm not the right person to advise you here as you need to ensure the weight doesn't exceed the loading the ceiling beams are designed for so some consultation with a builder / engineer might be necessary before starting any work.
