This is a very easy build and low cost as well at least for the enclosure. The aim is to use cardboard tubes to make a quarter-wave pipe. The simplest construction would be to simply block one end of a pipe and put a loudspeaker driver there. Here there is a trick were a smaller diameter tube is inserted into the larger one. The pipe is then in reality in three sections. 1. The smaller pipe, 2 the outer pipe minus the area of the inner pipe and 3 the outer pipe.
The inner diameter of the outer pipe (M) should be 1.5 times the effective cone diameter of the driver. The driver is a Phillips AD3701M a 6.5" driver with a cone diameter of about 130mm so 1.5 times that is 195mm and the pipe is 200 mm. Height of the outer pipe should be about 4 times its inner diameter or in this case 800 mm and that it what it is. The inner pipe inner diameter should be 0.6 M-wall thickness of inner-pipe so that is 200mm times 0.6 minus 4mm wall thickness = 116mm. As far as I can tell it is 118-120mm close enough. The average lenght of the inner pipe is 0.6 of the length of the outer pipe, in this case 480 mm.