left and right hand sides of the chamber 12 and a rear horizontal support 44 extends at an appropriate height from the rear vertical member 24 to the rear vertical member 19a two four inch wide plastic coated steel floor support plates 45a 45b extend between the front wooden supports 36 38 and 39 as shown and have upwardly extending right angle flanges 47 at their ends which are bolted to supports 36 38 and 39 as shown a generally p shaped substantially vertical rear steel support 46 four inches wide and having pluralities of pairs of apertures discussed in greater detail below therethrough has a vertical rear portion 48 see fig2 which is bolted to and extends upwardly from the rear support 22 to a point above the rear of the horizontal rear support 44 and is securely bolted in place to the rear of the support 44 a forward curved portion 50 extends integrally from the portion 48 upwardly over the support 44 and downwardly in front of the support 44 and has the bottom thereof bolted to the front of the bottom support 226 as will be described in greater below a plurality of bunji or shock cords 54 56 58 60 and 62 similar to those described in u s pat no 3 130.