You can have a team of up to six paddling, but more people = bigger raft = more weight = more flotation needed. You need around 100 litres of air (in sealed containers) for every person on the raft, plus more to allow for the weight of the raft itself. The usual way of doing this is with 100L plastic barrels that are cleaned and sealed airtight. These can be bought for around £10 each on eBay, Gumtree and other local classified ad websites.
The main rule is that your raft must be ‘obviously scratch built’, which effectively means it cannot be based on anything that was originally a boat, inflatable dinghy, canoe(s) or kayaks or any such other vessel.
Also consider: Where will you make your raft? Where will you store the raft? How will you safely transport your raft to and from the Lake Grounds?
The two basic designs are either a flat platform over sealed barrels, or cut out barrels that the crew sit or kneel inside. The first will sit higher in the water, and therefore have less drag, but can make paddling harder. The open barrel design sits lower in the water, which can help with paddling, but it is obviously a lot more prone to taking on water.
Whatever design you choose, your second concern (after staying afloat) is the comfort of the team. Paddling in a race is hard work, and anything that rubs, scrapes or chafes is quickly going to become a big issue!
Fancy dress is strongly recommended, and can win you a prize, but large flags or structures can massively slow you down or affect control of the raft if there’s any breeze. You cannot have a functional sail on your raft, but if you need one for a fancy dress theme then keep it minimal so that it does not slow you down. Also, don’t use a flag to display your raft number, for all the same reasons.
Portishead Raft Race is all about providing the opportunity for people to have great fun while raising much needed funds for good causes. You need to commit to raise a minimum of £100 for a good cause, so make sure your team choose one that will keep them motivated and chasing after sponsorship. Find out if your employer(s) will provide any sponsorship to enhance your fundraising.